The present invention relates to a ball bearing for use in a vacuum and a turbo-molecular pump having the bearing incorporated therein.
Grease-lubricated bearings having a cage are already known as ball bearings for use in turbo-molecular pumps.
Grease-lubricated ball bearings for turbo-molecular pumps are rotated at a high speed, for example, of about 75,000 r.p.m. during use, so that grease is not fully supplied to the pocket portions of the cage and the guide face of the bearing ring opposed to the cage. Further because the bearing is used in a vacuum, the frictional heat generated by the rotation of the bearing is not transferred to the outside effectively but remains in the interior of the bearing. Accordingly, the grease becomes degraded early, or early seizure occurs between the ball and the pocket portion of the cage or between the outer periphery of the cage and the inner peripheral guide face of the outer ring. Consequently, the life of the bearing is limited more greatly in a vacuum than in the atmosphere in view of the allowable temperature and becomes shorter. This leads to the problem of shortening the life of the turbo-molecular pump.